Bimetal thermostat element



Jan. 8, 1935. R. VALVERDE 1,987,167

BIMETAL THERMOSTAT ELEMENT Filed June 30, 193.3

a trZ" l/fi zrir INVENTOR ATTORNEY Patented Jan. 8, 1935 a UNITED STATES PATENT OFFICE 4 33:1 tzT-zjiz go. 678,349

This invention relates to bimetal thermostat is due to the generation of maximum stresses durelements, and more specifically .to that class ing the snap action. The bimetal thermostat which operates with a snap action when subjectelement of this invention has a central strip and ed to a change of temperature. two lateral strips so related and curved that they 5 Such thermostat elements obtain a snap action produce a snap action utilizing almost 100% of 5 by having portions of their surface constrained the possible stress intensity of the metal of the into lines which are not straight so that part or strips.

all of the surface is buckled. when heated the The central strip of my bimetal element is warping of the bimetal is opposed by the buckled loaded at both ends by the elastic lateral strips of surface until the warping stresses are sufllcient bimetal which confine the central strip of the 10 to cause the surface to buckle in the opposite slotted bimetal plate. The eflfect of change of Sense, This sudden ti i ll d a, snap temperature on such a system causes the lateral action. 4 Y strips to be stretched as the central strip warps The main purpose of these snap action bimetal from its flexed position to that of a straight colelements is to provide a thermostat which will u n. H w v in t s str ss sy m, longa- 15 snap at a given temperature, that is to say, the tiOn th lateral Strips is accompanied y a p m is control problem increase of load. Hence, as soon as the central One purpose of my invention is to obtain a snap strip warps beyond the lines of force of the lataction at given temperatures more readily than ral strip it buckles to an p e curve, re-

heretofore. lieving the compressive forces in the central strip 20 Another purpose is to obtain this snap action and the tension of t e ateral strips. Thus we temperature control without sacrificing the sudhave bimetal Surface, p b e of a S p action, den motion and large displacement of a usefu1 inwhich the maximum stresses, within the elastic snap action limits of the bimetal, may be set up to make this The position of unstable equilibrium taken by. -P action a um in velocity and displace- 25 my thermostat in the midst of the snap action mentis the snap position. Instead of controllably compressing the lateral The snap action obtained on heating is the heat strips to create the load, a simp ucture Ohsnap and that obtained on cooling is the cool Snap; tains by controllably expanding the central strip the resulting positions of rest are the heat snap in a special manner to be described. 30

and cool snap positions- The construction and operation of the inven- In my drawing, the position of the thermostat is tiOn Will be more d y understood y referring shown at various temperatures. to the o panymg drawing, in which,

Expanding parts of mybimetal elements means 1 is lJlan View f h preferred form. of increasing the longest dimension of the part my Slotted bimetal P e W an xpanded cen- 35 along its axis. This axis is the line passing tlal p; and through the centers of gravity of the crogs-sec- Figs. 2-4 are side elevations of the bimetal plate tions of the part. i of Fig. 1, showing the difierentpositions taken Where one end of the expanded central strip is y the Strips during a cycl f p ra ion.

' free there would be some temperature at which The thickness of the Plate and h rves are 40 the plate would regain its original flatness; this exaggerated in h drawing for clearness. The is the median temperature for that bimetalplate. lamination 7 W h h ater coefficient of ex- At this median temperature the plate can retain pansion is shown partly shaded at the ends of the either the heat snap 'or cold snap positions. side views.

The usefulness of a snap action thermostat in- The bimetal plate shown in Fig. 1 has two slots 45 creases directly with its ability to control. The 88 punched out to form, between dotted lines smaller the difference between the heat snap and 11 and 12, a central strip 9 and two lateral strips, cold snap temperature the better its ability to 10. Subsequent fabrication to produce my snap maintain a given temperature when operating action thermostat element is confined principally such apparatus as an electric heater, for instance. to the area between the dotted lines 11 and 12. 60 This diiference expressed asapercent of the tem- Holes 13 and 14 punched in the ends are for perature to be maintained is the regulation of mounting the plate and for attaching a feather the snap action thermostat. spring contact or other driven device.

The combination of good regulation with a The central strip 9 is expanded uniformly bepowerful snap action of considerable amplitude tween mutilated steel rolls of special design. As 55 the'plate feeds into these rolls it is deflected by a wedge shaped guide and given a slight overall curve, as shown in Fig. 2. A mutilated bead on one roll permits the lateral strips 10 and the end portions beyond 11 and 12 to pass through unexpanded while the gauge of the central strip 0 between 11 and 12 is reduced. with a plate having a thickness of .025 inch, this reduction may be about .0003 inch. The mutilated bead on the roll may have an elevation of. about .001 inch. The curved and expanded plate comes out of the rolls with the central strip bulging only a few thousandths of an inch above the lateral strips. The gauge of this bimetal is held between very close limits so that the amount of elongation of the central strip 9 can be accurately controlled.

Fig. 2 shows the bimetal plate at room temperature. The central strip buckles slightly above the lateral strips. The slight curve of the lateral strips is partly acquired from pre-forming the plate and partly caused by the end thrust of the buckled central strip where it Joins the end portions of the plate along the lines 11 and 12. All of the strips have a simple curvature. The terms simple curve and simple curvature are used in this specification to, indicate a curve which is free from reverse curves throughout its length. As the temperature of the plate rises, the bottom lamination, which has the greater coeiilcient of expansion, increases in length and causes the plate to flatten into the position shown in Fig. 8. The central strip is compressed and the lateral strips are put under tension as the unequal expansion of the laminations of the bimetal causes the strips to flatten. The importance of the simple curvature of the strips is apparent as the strips flatten because any reverse curve resulting in any substantial wave or kink in either the central or lateral strips would provide a point where the metal would bend without setting up the enormous compression and tension forces obtained with this invention.

In Fig. 3 the central strip is substantially flat and its upper and lower surfaces are substantially flush with the surfaces of the lateral strips. Elongation tests on the embodiment of the invention shown in the drawing, in which the central strip is wider and of greater cross-section than both of the lateral strips taken together, showa momentary increase in the total length of the plate caused by stretching of the lateral strips as the plate passes through the snap position shown in Fig. 3.

A further rise in temperature with continued unequal expansion of the laminations of the bimetal causes the plate to warp into an opposite curvature to that which it has at room temperature (Fig. 2). As soon as this opposite curvature begins, and the tension force of the lateral strips is no longer in line with the compression force of the central strip, these tension and compression forces causes the strips to snap into positions shown in Fig. 4. The compression in the central strip is relieved by the buckling of this strip into a curve extending below the lateral strips, and the relief of the compression force of the central strip automatically relieves the tension in the lateral strips.

If the temperature of the plate decreases, the lower lamination of the bimetal contracts more rapidly than the upper lamination and causes the plates to flatten toward the snap position shown in Fig. 3, and sets up similar compression and tension forces in the strips to cause the plate to snap back into the position shown in Fig. 2 as soon as it passes through the position of unstable equilibrium shown in Fig. 3.

Although the operation has been described with the snap occurring above room temperature, the snapcan bemade tooccurat anytemperatureby giving the plate the correct initial curvature. For example, if the plate were in the pcsitionshown inl'lg. 4atroomtemperature,thesnapwould occur below room temperature. i

This application is a continuation in part of application Serial No. 894.104. filed September 20, 1920.

I claim as my invention:

1. A one-piece slotted bimetal plate integrally forming a central strip and two lateral strips, said central strip being longitudinally expanded withrespecttothelateralstripstnfm-masimple developable curve. x

2. A thermostat element comprising a onepiece slotted bimetal plate forming a central strip and two lateral stripe, said central strip being expanded at a given temperature, along its axis sothatsaidcentraletrlpatthegiventempem tureislongerthantheplatebetweentheendsd saidcentralstrip,andwherebyeaid centralstrlp iscausedtofiexbeyondthesurfaceoftheplate in a simple developable curve.

3. A slotted bimetal plate integrally forming a central strip and two lateral strips, said central strip having a cross-section greater than the combined cross-sections of said lateral strips, and said central strip being expanded longitudinally team it to take the form of a developable surface of simple curvature.

4. A bimetallic thermoresponsive plate having slots therein forming a central strip and two lateral strips, the said central strip having an initial permanent stress along its operative length, whereby said central strip is bowed into a surface of simple curvature. said two lateral strips being normally shorter than the central strip to maintain the stress in said central strip.

5. A curved bimetallic slotted plate forming a central strip and two lateral strips, said central strip being expanded longitudinally to form a developable surface of simple curvaturawhich surface on heating becomes substantially ilush with the surface of the plate, said central strip becoming substantially flat before buckling with a snap action into a position of oppomte curvature.

6. A curved bimetallic slotted plate forming a central strip and two lateral strips, said central strip having a developable surface of simple curvature, which surface on changeof temperature buckles with a snap action to an opposite curvature, all curved surfaces of the plate being curved toward the same side at any given time.

7. A one-piece slotted bimetal plate integrally forming a central strip and two lateral strips, said central strip being expanded longitudinally to form a developable surface of simple curvature, and of greater length than said lateral strips so that the central strip is under longitudinal compression maintained by tension of said lateral strips, the tension in said lateral strips and the compression in said central strip increasing gradually with change of temperature until a snap action occurs.

8. In a thermal responsive device, a one-piece bimetal plate having lateral side strips and a central strip separated from the side strips by parallel slots which terminate short of the ends of the plate, the length of the central strip being greater than that of the side strips, and all portions of the plate having a simple curvature in the same direction when the plate is below-its median temperature.

9. In a thermal responsive device, a one-piece bimetal plate having a. central strip, and side 5 strips separated from the central strip by slots which terminate short of the ends of the plate, the central strip being of a diflerent length than the side strips, all of said strips being bowed in the same direction when below the median temperature oi. the plate and being free from reverse curves so that the forces which cause snap action or the. plate, when it passes through its median temperature, are compression and tension stresses in the strips;

ROBERT VALVERDE. 

